How can some ammunition be more accurate than others?

There are several things that make rounds more consistent. What you want is every bullet traveling at the same speed and flying the same path. Different volumes of the inside of the case can cause the powder to develop more or less pressure at different rates. Different powders burn at different rates is the case depending upon the characteristic of the powder and the fullness of the case. The concentricity of the neck of the case holding the bullet. The uniformity of the crimp on the neck of the bullet. The consistent seating depth of the bullet in the case. And the symmetry and matching weights of all of the bullets. There are numerous other things that I have left out but this covers most of them, Oh, one other the consistency of the primers so that the powder is ignited the same in each cartridge.